The wastewater containing organic pollutants is usually treated using a biological process. The suspended-growth process, which is also known as the activated sludge process, is one of the most widely used biological processes. For example, most municipal wastewater treatment plants employ the activated sludge process in their secondary treatment stage for removing organic pollutants from the wastewater. The conventional activated sludge process comprises a suspended-growth bioreactor (conventionally referred as the aeration tank when operated in aerobic conditions) and a detached clarifier (conventionally referred as the secondary clarifier). The wastewater and the return activated sludge from the clarifier flow into the aeration tank. Air or oxygen is supplied to the aeration tank through an aeration system. In the aeration tank, pollutants are either degraded or adsorbed by the activated sludge. The aeration tank mixed liquor then enters the secondary clarifier for solid-liquid separation. The supernatant of the secondary clarifier is discharged through the clarifier outlet. Most of the settled sludge in the clarifier is returned back to the aeration tank. Excess sludge is wasted to a sludge handling system for further treatment. Wasted sludge or high concentrated wastewater can be treated using anaerobic method to produce biogas while reducing pollutant load. The fixed-film process, which uses fixed or moving media to retain microorganisms, have also been widely used for wastewater treatment. The fixed-film process normally does not rely on the sludge return from the secondary clarifier to maintain appropriate amount of biomass for wastewater treatment.
In most cases, the wastewater also contains organic nitrogen, ammonia, and phosphorus. They are called wastewater nutrients because they can cause the excessive growth of algae in the receiving water body, e.g. eutrophication, significantly impacting the surface water quality. In addition, the organic nitrogen and ammonia consume oxygen in the receiving water body during their oxidation. These wastewater nutrients can also be removed in the bioreactor. Microorganisms can convert organic nitrogen and ammonia to nitrate or nitrite under aerobic conditions. This process is called nitrification. If the bioreactor or part of the reactor is under anoxic conditions (no dissolved oxygen (DO) presents), microorganisms can reduce the nitrate and nitrite to nitrogen gas. This process is called denitrification. If the bioreactor is maintained in low DO aerobic conditions, simultaneous nitrification/de-nitrification can be achieved. If the aerobic sludge continuously passes through an anaerobic zone then an aerobic zone in the bioreactor, a group of microorganisms favorable for phosphorus uptake can be acclimated.
The combination of nitrification/denitrification processes can be achieved in a number of ways. The conventional method includes a bioreactor and a secondary clarifier. The bioreactor includes two zones or two individual tanks: an aerobic zone/tank for nitrification, and an anoxic zone/tank for denitrification. Activated sludge is returned from the clarifier to the bioreactor to maintain a certain amount of biomass for nitrification and denitrification. If the anoxic zone is ahead of the aerobic zone, it is called a “pre-anoxic” process. For this process, organic matter in the influent is used as the electron donor for denitrification, thereby removing some organic matter during denitrification. However, this process relies on the return of final sludge and/or mixed liquor to provide nitrate to the anoxic zone. Therefore, only the nitrite/nitrate contained in these return streams can be removed. A certain fraction of the nitrate/nitrite in the aerobic zone (depending on the return ratio) is never returned to the anoxic zone, which limits the extent of denitrification. If the aerobic zone is ahead of the anoxic zone, it is called a “post-anoxic” process. This process cannot use influent organic carbon for denitrification. Therefore, the denitrification rate is generally very slow and an external carbon source is usually added to promote denitrification. Carbon addition increases operational complexity and cost.
The step-feed/step-aeration process is also used to perform nitrification and denitrification. In this process the bioreactor is separated into several sequential anoxic/aerobic sections. Aeration is provided in aerobic sections to perform nitrification. However, raw wastewater is fed into each of the anoxic sections and mixed with the nitrified mixed liquor from the preceding aerobic section for denitrification. This process can use the organic matter in the raw wastewater for denitrification. However, sludge return from a secondary clarifier to the first anoxic zone is needed to provide sufficient biomass for both nitrification and denitrification.
There is also an alternating anoxic-aerobic (AAA) process for total nitrogen removal. In this process the bioreactor is not separated into different sections, but rather creates aerobic and anoxic conditions within the same volume at different times. Aeration is applied to create the aerobic condition, and nitrification/organic matter removal are accomplished. Aeration is then ceased and anoxic condition begins. During the anoxic condition inflow commences, and denitrification is performed. Again this process requires a secondary clarifier for solids-liquid separation and a separate sludge return system to seed the bioreactor for biological reactions.
The simultaneous nitrification/denitrification process is also used to perform nitrification and denitrification within one tank. In this process, the entire tank is maintained under a low DO condition so that anoxic conditions can be maintained inside the flocs of activated sludge, allowing the nitrate/nitrite that has diffused into the flocs to be denitrified. However, it is not easy to maintain precise DO concentrations, and a complex control system must be used. In addition, low DO reduces the rate of nitrification. This process also requires a secondary clarifier to perform solids-liquid separation and a separate sludge return system to seed the bioreactor.
The sequencing batch reactor (SBR) can achieve nitrification, denitrification, and solids-liquid separation within one tank. During the aeration period nitrification occurs, while denitrification occurs during the feeding and mixing period. Sludge is settled and retained within the same tank during the settling period. However, after nitrification a fraction of the nitrate in the supernatant must be decanted to allow a new feeding cycle to begin. The effluent nitrate concentration is dependent on the influent total nitrogen concentration and the fraction of feed volume to total tank volume in one cycle. Therefore, only the portion of nitrate in the tank after decanting can be denitrified. Due to the use of the mechanical decanting system inherent to the SBR process, frequent but small volume decanting and feeding, which is essential to reduce the final effluent nitrate concentration, is not possible; therefore the total effluent nitrate concentration cannot be maintained at desirably low level. Moreover, the decanting process uses many mechanical moving parts, all of which can be problematic for operation.
To remove both nitrogen and phosphorus, more complex processes have to be used. These processes include an anaerobic zone to culture phosphorus accumulating organisms (PAOs), an anoxic zone to denitrify nitrate and nitrite, and an aerobic zone to oxidize organic pollutants and perform nitrification. Sludge is returned from the secondary clarifier to the bioreactor for needed biodegradation reactions.
FIG. 1 shows a conventional pre-anoxic process for total nitrogen removal. It has an anoxic zone for denitrification followed by an aerobic zone for organic matter degradation and nitrification. Mixed liquor in the aerobic zone is forcibly returned to the anoxic zone to provide nitrate. The effluent from the aerobic zone flows through a secondary clarifier for solids-liquid separation, and settled sludge in the secondary clarifier is returned to the anoxic zone to provide appropriate amount of biomass needed for biological functions. Supernatant in the secondary clarifier is discharged. The anoxic zone is continuously mixed, mostly through mechanical mixing devices.
FIG. 2 shows a conventional step-feed process for comprehensive nitrification and denitrification. It includes several sections or zones that alternatively perform denitrification and nitrification. Similar to the pre-anoxic process, it has a separate secondary clarifier and sludge is returned from the secondary clarifier to the first anoxic zone, and all anoxic zones are continuously mixed, mostly through mechanical mixing devices. The influent is fed to multiple anoxic zones to reduce the amount of nitrate produced in the following aerobic zone, and to provide carbon source for denitrification. This process can achieve better total nitrogen removal.
FIG. 3 shows a conventional anaerobic-anoxic-oxic (A2O) process for total nitrogen and phosphorus removal. It has an anaerobic zone for culturing PAOs, an anoxic zone for denitrification, and an aerobic zone for organic matter degradation and nitrification. Mixed liquor in the aerobic zone is forcibly returned to the anoxic zone to provide nitrate. The effluent from the aerobic zone flows through a secondary clarifier for solids-liquid separation, and settled sludge in the secondary clarifier is returned to the anaerobic zone to provide appropriate amount of biomass needed for biological functions. Supernatant in the secondary clarifier is discharged. The anaerobic zone and anoxic zone are continuously mixed, mostly through mechanical mixing devices. The bioreactor used in the University of Cape Town (UCT) process also has three zones, however, the activated sludge in the clarifier is returned to the anoxic zone to remove nitrate, and the denitrified mixed liquor from the anoxic zone is returned to the anaerobic zone to culture PAOs. Compared to the conventional A2O process, the UCT process adds one more return stream but maintains a better anaerobic condition in the anaerobic zone.
FIG. 4 shows a bioreactor such as is disclosed in U.S. Pat. No. 6,787,035 that has been designed with an internal settling device (24, 26, 28, 30) to automatically return sludge to the aerobic zone (18). This system uses an aerobic zone (18) for organic matter removal and nitrification, and returns a portion of the liquor to an open bottom pre-anoxic zone (16) for denitrification. Supplemental sludge is returned from final clarifier (36) back to the bioreactor through a sludge return device (38). During normal operation, influent is continuously fed to the bioreactor and the aeration device (22) is continuously operated to charge oxygen to the bioreactor.
Anaerobic digesters have been used in many areas of the world to produce biogas for cooking, heating, and electricity using human and animal wastes, high strength wastewater, and sludge. The major component of an anaerobic digester is a sealed tank. This tank receives and digests organic matter under anaerobic conditions. During digestion microorganisms convert the organic matter to biogas after several metabolic steps. The key difference between a high-rate anaerobic digester and a conventional anaerobic digester is mixing. Appropriate mixing can significantly improve the digestion performance because it provides better contact between the microorganisms and the organic materials, prevents the sludge build up, and breaks apart floating sludge. For large installations, high-rate anaerobic digesters are normally used. A number of mixing methods such as mechanical mixing and gas mixing have been applied. These mixing types usually need external energy input and periodic maintenance. For example, mechanical mixing requires impellers and motors. Gas mixing, although relatively mild, still requires a gas compressor to recycle the biogas to the bottom of the tank. For small installations (such as those used in households and small communities), however, it is not cost-effective to employ these mixing methods. In particular, the application of these mixing methods is not possible in regions where there is no electricity. As a result, only bulky conventional anaerobic digesters, which do not have deliberated mixing systems, are used as biogas generators.
The effort to install conventional, non-mixed anaerobic digesters for small installations is significant. The key roadblock for mass implementation of these conventional digesters is their large size. Large tank volumes require large footprints and significant cost for construction, and these tanks need to be constructed onsite in most cases. Large tanks are also prone to leaking—and biogas leaking is the primary cause of biogas generator failure. Large tank designs are required because of the low reaction rate due to the lack of appropriate mixing. Only very mild mixing exists, caused naturally by the rising of small biogas bubbles.
While some past iterations of anaerobic digesters have relied on propeller-type mixing devices inserted into the tank, prior art shows improvements to mixing within anaerobic digesters using draft tube mixing units. The draft tube mixing unit typically contains a self-contained, propeller-type agitator that induces flow from the top of the tank, just below the liquid's surface, to the bottom of the tank. If more than one draft tubes are utilized in a single tank then the outlets of the draft tubes are aligned in a way as to induce a vortex with in the reactor. This provides two crucial functions: first, as previously mentioned, turbulence within the reactor increases contact frequency between microbes and substrate, increasing metabolic activity and gas production; secondly, agitation of the surface can break apart floating sludge and reintroduce it to the mixture. Too much floating sludge can create operational issues for anaerobic digesters including decreased gas production and clogging of effluent pipes.
High-efficiency, completely-mixed anaerobic digesters have a smaller reactor size for the same biogas yield. A portable anaerobic digester capable of high-efficiency anaerobic digestion typically has components of similar reactors (i.e., influent pipe, effluent pipe, sludge wasting pipe, etc.). Such reactors may use a single impeller or multiple impellers to lift solids from the bottom of the reactor and distribute them across the top of the reactor, which also has the effect of breaking apart any floating sludge. Other types of common mixing devices may also be used, such as a draft tube, injected gas, vacuum pumping, mixing blades and the like. The effluent port is typically positioned below the level of the fluid to minimize clogging occurrence as the result of floating sludge. Although this type of reactor is able to achieve higher biogas generation per volume of reactor over conventional non-mixed designs, the net energy output of the reactor is reduced due to the energy input needed to drive the mixing mechanism.
Fluids or fluid-like substances are often transported against gravity by the use of mechanical devices that provide positive and negative displacement (e.g., diaphragm pumps) or that apply kinetic energy directly to the fluid (e.g., centrifugal pumps). These types of devices often have many mechanical moving parts and, therefore, require significant amounts of maintenance.
Traditional airlift pumps can also be used to move and mix fluids. The traditional airlift pump has several advantages over mechanical pumps in that they generally have no moving parts in the pump that can fail due to mechanical wear. An air source provides the driving force in the pump, allowing for easy or no pump maintenance. Furthermore, airlift pumps are robust, light, and easy to install and transport compared to their mechanical counterparts. In a traditional airlift pump, when gas is introduced into a riser the density of the fluid in the riser is decreased, allowing for liquid and solids transport from the bottom to the top of the riser.
Conventional airlift pumps have disadvantages as well. Perhaps the most significant is the inability to apply a great deal of head or pressure to the fluid. In addition, airlift pumps are limited by relatively small pump housing diameters therefore may not able to achieve high flow rates. If the pump housing of an airlift pump has a large diameter, than the air bubbles within the housing are relatively more dispersed and can not form large bubbles within the housing. Therefore, lifting force is reduced with an increase of the pump housing diameter.
If there is a method and apparatus that can form large air or gas bubbles within the pipe to lift the liquid, the pump performance would be improved. In addition, the pump housing diameter can be increased without losing lifting force, thus achieving higher flow rates. The intensive lifting force caused by the large air or gas bubble can also be used for mixing the fluid within various types of reactors. Some methods for improving the efficiency of airlift pumps do so by introducing air to an airlift pump so as to allow the gas to accumulate in a volume under the liquid surface. Once the gas reaches a predetermined volume a large bubble of gas enters the pump riser through an orifice. Such devices may be thought of as “surge lift” devices as they collect a predetermined volume of gas and release it in a single “surge” to improve performance. The large bubble expands as it rises due to decreasing fluid pressure. As the bubble expands it fills the entire riser, creating a much greater force than the small bubbles in a traditional airlift pump. In other methods a gas supply line has been added to allow the pump to operate as a traditional airlift pump between large-bubble surges, effectively increasing overall flow rate. All of these previous methods for increasing the efficiency of an airlift pump include an elbow-shaped means of introducing the air from the air chamber to the riser. In certain applications this means of air introduction could become clogged and result in pump failure.